Electrodeposition has become the primary method for applying corrosion-resistant primers in automotive applications. Advantages of electrodeposition over non-electrophoretic coating processes include increased paint utilization which reduces raw material and waste disposal costs, as well as improved corrosion protection.
The electrodeposition process involves immersing an electroconductive substrate into a bath of an aqueous electrocoating composition, the substrate serving as a charged electrode in an electrical circuit comprising the electrode and an oppositely charged counter-electrode. Sufficient electrical current is applied between the electrodes to deposit a substantially continuous, adherent film of the electrocoating composition onto the surface of the electroconductive substrate.
The electrocoated substrate is then conveyed to a rinsing operation where it is rinsed with an aqueous rinsing composition. Typical rinsing operations have multiple stages which can include closed loop spray and/or dip applications such as are described below. For example, in a spray rinse application the electrocoated substrate exits the electrocoating tank and is conveyed over a rinse tank while an aqueous rinsing composition is spray applied to the electrocoated surfaces of the substrate. Excess rinsing composition is permitted to drain from the substrate into the rinse tank below. The rinsing composition is then recirculated to the spraying apparatus for subsequent spray applications. In a dip rinse application, the electrocoated substrate is conveyed into a dip tank, where it is immersed in an aqueous rinsing composition, and is subsequently conveyed through one or more spray rinse applications as described above.
Recirculating the coating or rinsing compositions is both economically and environmentally desirable. However, the combination of organic nutrients, warmth and recirculation in an aqueous coating system creates an environment conducive to bacterial and fungal growth. These microorganisms, if left unchecked, can adversely affect the quality and appearance of the electrodeposited coating. Microorganisms present in the coating or rinsing compositions can cause pH shifts, particulate "dirt" deposition and biofouling, which detrimentally affect the appearance of the coating and reduce system performance.
Ethylene glycol ether alcohols can suppress microorganism growth in electrocoating compositions, but are undesirable ecologically. Propylene glycol ether alcohols, often used in HAPS-free (Hazardous Air Pollutant-free) electrocoating compositions, are more ecologically desirable but do not adequately suppress microorganism growth.
A microbiocide composition containing a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one.sup.1 has been used commercially in electrodeposition coatings and rinse compositions as the sole microorganism control composition. Although effective for inhibiting and/or controlling the growth of microorganisms in such systems, this microbiocide is relatively expensive and can cause a rougher appearance than a coating composition without this microbiocide. Moreover, such microbiocide compositions can contain, as inert ingredients, metal salts such as magnesium nitrate and magnesium chloride. The presence of metal ions of these salts in electrodeposition systems is undesirable because the metals cause coating defects such as gas generation at the cathode.
 FNT .sup.1 KATHON.RTM. LX microbiocide which is a mixture of 10-12% of 5-chloro-2-methyl-4-isothiazolin-3-one, 3-5% of 2-methyl-4-isothiazolin-3-one, 14-18% magnesium nitrate, 8-10% magnesium chloride and the remainder water. "KATHON.RTM. LX microbiocide", Material Safety Data Sheet of Rohm and Haas Co. (Jun. 24, 1993), which is incorporated herein by reference.
U.S. Pat. No. 4,732,905 discloses biocidal compositions for inhibiting and/or controlling the growth of bacteria in aqueous systems, for example, cooling water and pulp and paper mill systems. The disclosed biocidal compositions comprise a synergistic mixture of 2-bromo-2-nitropropane-1,3-diol and a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one. The patent does not disclose, however, the use of these biocidal compositions in electrodeposition systems.
EP 608914 discloses the use of a synergistic microbiocidal composition comprising 4,5-dichloro-2-octyl-3-isothiazolone and 2-bromo-2-nitropropanediol in aqueous paints and coatings, including electrodeposition systems. Although effective in controlling microbial growth, this composition is relatively expensive and can contain undesirable metal ions such as are discussed above.
Therefore, there is a need for electrodeposition and rinsing compositions which provide smooth coatings of good appearance, minimize growth of microorganisms and which do not contain metal ions which need to be removed prior to waste disposal.